First Principles Investigation of Binary Chromium Carbides Cr7C3, Cr3C2 and Cr23C6: Electronic Structures, Mechanical Properties and Thermodynamic Properties under Pressure

Binary chromium carbides display excellent wear resistance, extreme stiffness and oxidation resistance under high temperature. The influence of applied pressure on electronic structure, elastic behavior, Debye temperature and hardness of Cr7C3, Cr3C2 and Cr23C6 have been investigated by the density functional theory (DFT) method. The results reveal that lattice parameters and formation enthalpy display an inverse relationship with applied pressure, and Cr3C2 exhibited optimal structural stability. Moreover, Cr-C orbital hybridization tends to be stronger due to the decreased partial density of states (PDOS) of the Cr atom. The difference in electronic distribution of binary carbides has also been investigated, which confirmed that overall orbital hybridization and covalent characteristics has been enhanced. The theoretical hardness was elevated according to the higher bond strength and bond density. In accordance with structural stability data, Cr3C2 has shown maximum theoretical hardness. Furthermore, the anisotropic nature of hardness has been evaluated with external pressure. Cr3C2, and the highest isotropic hardness behavior along with an increase in hardness values with increasing pressure has been observed. In addition, the variation in Debye temperatures of binary chromium carbides under applied pressure has also been predicted. The results provide a theoretical insight into electronic, mechanical and thermodynamic behavior of three binary chromium carbides and show the potential of these novel carbides in a wide range of applications.

Influence of Heat Treatment on Structure and Durometric Properties of Coatings Obtained by Surfacing with CSR-04СR27NI7MO3CU2Т Cast Rods

The effect of heat treatment modes on the structure and durometric properties of coatings obtained by surfacing with CSR-04СR27NI7MO3CU2Т cast rods, is considered. It is found that the temperature of 800°С and soaking time of 5 hours are optimal to increase the deposited metal hardness. It is shown that such a phenomenon results from the formation of the austenitic structure hardened by the precipitates of the σ-phase (FeCr), chromium carbides (Cr3C2) and titanium carbides (TiC). The heat treatment modes proposed can be applied in the wear-resistant surfacing technology of chemical equipment parts.

Forming of Composite Cutting Layer on Tool Steel by Heat Treatment of CVD Coatings

СVD chromium coatings are evaporated on steel substrate from chrome-organic compounds. For crystallization with forming of nano-particles of chromium carbides, subsequent heating (annealing) of tool steel with hybrid coatings is carrying out. Significant increase of micro-hardness of the coating up to 27000 MPa is observed due to the dispersion strengthening. Optimal annealing parameters (temperature and duration) are determined, which maximally strengthen the coatings and increase their adhesion to the steel substrate.

Increase of Stainless Steel Wear Resistance by Diffusion Chromium Plating Using Iodine Transport

The article presents the results of a study of diffusion chromium plating of stainless steel X20Cr13 by iodine transport. The main kinetic laws of the process - the effect of temperature and time of chromium plating on the thickness of the coatings - have been studied. It is shown that the main phases of the coating are chromium carbides and nitrides with surface microhardness HV up to 15 GPa. The tribotechnical properties of the obtained coatings on rollers - friction pairs are determined.

Влияние дополнительной термической обработки газофазных неоднородных поверхностных слоев на остаточные напряжения и адгезионную прочность стали

The article presents the results of a study of the surface layers obtained by the deposition of organometallic chromium compounds on the base metal, and the structural changes occurring in them when exposed to additional heat treatment in the form of subsequent annealing. It is noted that the type of coating and its structure primarily depend on the deposition temperature; therefore, there are three main types - kinetic, transitional and diffusional. Each such type of surface layer is determined by its own mechanism of formation, and as the temperature changes, one transforms into another. The metastable state of such surfaces, which are supersaturated solid solutions, upon additional heat treatment leads to significant structural changes and the formation of a nanostructured composite layer. The possibility of obtaining nano-modified layers by forming the structure of a matrix solution with dispersed-dissolved secondary nano-sized particles of chromium carbides is shown. The modes at which crystallization of the amorphous phase begins with the release of nano-sized particles of chromium and chromium carbides have been determined. The modes with the maximum and minimum levels of residual stresses, which directly affect the performance of products, have been established, and the effect of additional annealing on the distribution of residual stresses has been studied. It is shown that the process of crystallization of the amorphous phase during annealing and the parallel process of coagulation of carbides causes the appearance and growth of residual stresses. The influence of residual stresses on the adhesion strength of surface layers, which largely depends on the main parameters of heat treatment - temperature and time, has been investigated. Comparison of adhesion strength before and after annealing showed that the overall level of adhesion strength during heat treatment of coatings decreases insignificantly, by 7-10%.

CHROMIUM CARBIDES AND CYCLOPROPENYLIDENES

Carbon tetrabromide can be reduced with CrBr2 in THF to form a dinuclear carbido complex, [CrBr2(thf)2)][CrBr2(thf)3](μ-C), along with formation of [CrBr3(thf)3]. An X-ray diffraction study of the pyridine adduct displayed...